Gas turbine engines are utilized for a large variety of purposes including propulsion by thrust, propulsion by mechanical coupling, driving accessories requiring a rotary input, providing compressed air, and combinations thereof. The compressed air provided may be taken from a load compressor driven by the turbine or as bleed air. The latter is known as "bleed air" because it is bled from the turbine engine at some location following partial or total compression by a rotary or centrifugal compressor utilized in such engines to compress air to be used to sustain combustion. Bleed air may be utilized for a variety of purposes. For example, in an aircraft, it may be utilized for cabin ventilation, de-icing, driving an air motor for starting main propulsion engines, and the like.
In any event, many of the uses to which bleed air is put are variable in the sense that the quantity of bleed air required for a given use will vary over a period of time. At the same time, the demand for air to support combustion for operation of the turbine engine will be more constant although it, also, may vary dependent upon the load placed on the engine where the engine is mechanically coupled to a pump or a generator or the like.
As a consequence, a decrease in the demand for bleed air, without more, can result in so-called compressor surge or back flow that will occur because of the presence of a higher pressure in the combustor for the engine than in the diffuser for the combustor. As is well-known, surge is highly undesirable as it causes unstable operation of the turbine engine and can cause substantial damage to compressor components.
To avoid this difficulty, the prior art has resorted to the use of, for example, surge protection valves which are operable to open a flow path through which bleed air in excess of that demanded (sometimes referred to as "excess pneumatic load") at a particular time may be dumped to prevent compressor surge. This method of providing surge protection is satisfactory in preventing surge from occurring, but requires that the turbine engine operate for a greater period of time at or near a full load condition. The relatively high loading that results reduces engine life and in addition, consumes unnecessarily large quantities of fuel.
In the above-identified applications, the disclosures of which are herein incorporated by reference, a gas turbine engine having unique characteristics allowing the same to provide variable amounts of bleed air while avoiding surge related problems is disclosed. In particular, the above-identified applications disclose a gas turbine engine wherein the load compressor is integrated into the engine so that the engine has first and second rotary compressors which are driven by a turbine wheel. Bleed air is taken between the interface of the first and second stage compressors and variable inlet guide vane geometry is employed at the inlet of the first stage compressor. In addition, the first stage compressor is a so-called high specific speed compressor. High specific speed (N.sub.s) is equal to or greater than about 100 where it is defined as ##EQU1## and N=rpm of the first stage compressor,
CFS=first stage compressor inlet volumetric flow rate in ft.sup.3 /second, and
H.sub.ad =adiabatic head in ft.
In addition, it is preferable that the first stage compressor have an impeller blade tip angle greater than zero degrees from the radial direction and be followed by a vaned diffuser in the first stage compressor outlet.
As pointed out in the above-identified applications, it is possible to operate such a gas turbine engine on the stable side of the surge line simply by varying the inlet guide vane geometry appropriate to bleed air flow allowing the turbine to be fueled only as required to meet the actual demand for power. Consequently, the problem of surge is minimized or eliminated altogether while fuel consumption is reduced as is engine loading. In short, the gas turbine engine disclosed in the above-identified applications provide surge protection without wasteful dumping of excess bleed air and/or operation near or at full load conditions.
The present invention is directed to providing an optimized control system for such an engine.